Most metropolitan areas are now equipped with one or more forms of wireless communications networks which provide radio transmissions in the microwave frequency band to wireless telephone customers. For example, a plurality of base transceivers capable of transmitting and receiving radio communication signals to and from one or more portable handsets are located throughout various wireless service areas. A portable handset, as the term is used herein, refers to a wireless voice telephone such as a cellular telephone or a cordless telephone or the like in a speech or data mode that can be installed at a fixed location in an indoor environment, temporarily installed in a vehicle or be completely mobile. In the case of cellular telephones, a typical cellular service area comprises a metropolitan area or larger region.
An example illustrating two possible configurations of a cellular service area comprised of segments 10a, 10b is shown in FIG. 1. The segment 10a illustrates a seven-cell reuse pattern; i.e., the frequency band allocated to cellular telephone customers is divided into seven macrocells 12 operating at seven different frequency channels F1-F7. The frequency channels F1-F7 typically are not consecutive frequencies of the cellular radio frequency band. For example, frequency channel F1 may comprise channels 1, 3 and 5, while frequency channel F2 may comprise channels 2, 4 and 6. The seven frequency channels F1-F7 are repeated for each segment 10a of the entire cellular service area. The cellular service area 10b illustrates a twelve cell site reuse pattern.
Service area 10b has reduced capacity per coverage area since the cellular frequency band is divided into twelve macrocells 12, and hence twelve frequency channels F1-F12, as opposed to the seven frequency channels F1-F7 of segment 10a. However, the larger segment 10b provides a greater distance between macrocells 12 that use the same frequency channel, resulting in reduced co-channel interference, e.g., frequency channel F6 of segment 10b versus frequency channel F6 of segment 11b.
Located within each macrocell 12 is a base transceiver 14. Each base transceiver 14 is able to broadcast and receive radio signals utilizing its corresponding frequency channel. For example, a base transceiver 14 located in segment 10a would transmit and receive radio signals from a wireless transceiver utilizing frequency channels F4 while adjacent base transceivers 14 would operate at frequency channels F1, F2, F3, F5, F6 and F7.
When a telephone call to a wireless transceiver originates from either another wireless transceiver or a land-based telephone via a Public Switched Telephone Network (PSTN) 15, a caller must access a Mobile Telephone Switching Center (MSC) 16. The MSC 16 receives the call request and instructs a central call processor 17 to begin call processing. The central call processor 17 transmits a signal over a dedicated line 18 (such as a land-based line or microwave link, etc.) to a predetermined base transceiver 14. The central call processor 17 determines which base transceiver 14 to transmit the signal based on the location of the portable handset. When the portable handset enters a macrocell 12, the portable handset registers its location with a corresponding home or visitor location register. The control call processor 17 has access to the registers and, therefore, directs the telephone call to the appropriate base transceiver 14.
Portable handsets can be used indoors as well as outdoors. Therefore, a portable handset located in an indoor environment requires service from a base transceiver 14 located outdoors. The indoor portable handset can communicate directly with an outdoor base transceiver 14 if both the base transceiver 14 and the portable handsets are allowed to exert sufficient power to penetrate the walls of a building. However, the more power a base transceiver 14 or a wireless transceiver transmits the more interference it causes to other base transceivers 14 as well as the associated portable handsets which are assigned the same frequencies.
Consequently, a need has developed for a wireless transmission system and, in particular, wireless local loops or links (WLL) which utilize the attenuation properties of buildings within their associated service areas. A WLL is a wireless link which connects an indoor cellular portable handset or a customer premises wired standard telephone to an outdoor cellular base transceiver on demand. Such a system should permit the buildings to act as shields so as to isolate outdoor users as well as the user's respective base transceiver 14 from interference caused by the indoor WLL. Still further, the WLL should allow users to make inexpensive local calls using the same portable handsets which were used to make expensive outdoor calls. Further, the WLL should provide indoor communication quality for users of outdoor portable handsets when the users transfer to indoor environments.
Indoor coverage by an outdoor base transceiver requires a device that compensates for most of the indoor propagation loss due to the attenuation properties of the building. The indoor propagation loss can be as high as 90 dB. A device such as an on-frequency repeater can be utilized to recover the indoor propagation loss if the repeater's directive antennas are professionally installed and reflectors are not located in close vicinity to the repeater's antennas. This requirement can rarely be guaranteed when a consumer-installed Customer Premises Equipment (CPE) is utilized.
One known method of improving indoor coverage for portable handsets includes utilizing an expensive Frequency Shifting Repeater (FSR) that resembles two back-to-back base transceivers. The first base transceiver receives a signal from the outdoor base transceiver, demodulates the signal to a baseband form including error correction and speech processing, and then sends the signal to the second base transceiver that performs a reverse process. Finally, the signal is sent by the second base transceiver to the indoor portable handset. The double baseband processing by this type of FSR doubles an already existing speech delay. This type of FSR is expensive and, therefore, is not feasible for use in a mass market application.
A second known method utilizes an inexpensive FSR which does not possess any demodulation or error correction/speech processing capabilities. A base transceiver transmits a signal at a first frequency and the portable handset receives the signal at a second frequency. In order to avoid co-channel interference, the second frequency assigned to indoor operations must be different from the first frequency assigned to outdoor operation.
The portable handset may receive a message on the second frequency to move to a traffic channel (time slot) on the first frequency and attempt to do so. Since the indoor power of the signal on the first frequency is very low due to the attenuation property of the building, the portable handset will attempt to report that condition to the base transceiver. It is possible that the base transceiver will not receive the message; however, if the base transceiver does receive the low power message, the base transceiver will assign another of its channels to the portable handset. Since none of the base transceiver's channels operate on the second frequency, the assignment will not succeed.
As referenced above, the known prior art fails to disclose an inexpensive cellular WLL. Consequently, a need has developed to provide a wireless local loop system and method as referenced above which may be practically and economically implemented for use by single and multi-family dwellings and multi-use building environments.
It is further desirable that such a system be compatible with existing digital cellular radio communications. Finally, such a system should neither require the allocation of more radio frequencies than are currently allocated to wireless communication systems, nor require a substantial portion of existing wireless frequencies.